1. Field of the Invention
This invention relates to image-capturing devices; specifically to rotating-mirror-based streak cameras.
2. State of the Art
Streak cameras capture a thin line view of the subject continuously over time, thereby creating a record of one dimension of space over time. The most common type of streak camera is an image converter based camera, which has the advantage of being able to capture images with extremely high temporal resolution in the picosecond and sub-picosecond domain. It functions by converting incident photons from an image into electrons and then electro-statically sweeping these electrons, and then capturing the resulting transient record.
Another, less common type of streak camera is rotating-mirror based. Rotating mirror streak cameras are an indispensable tool for the diagnostics and analysis of complex, rapidly moving events, including high explosive events. While operating at slower speeds than the image converter cameras, having temporal resolution on the order of nanoseconds or microseconds, they have the advantage of longer record length and significantly higher spatial resolution, on the order of 5,000 picture elements or more. They operate by optically relaying the line image through a very fast rotating mirror, which has an axis of rotation parallel to the line image. The mirror is positioned at a location distant from the focal points of the optical system, which as the mirror turns results in a sweeping of the focused line along a surface that is roughly cylindrical (exactly cylindrical if the thickness of the mirror is neglected). The rotating mirror is then roughly at the center of this cylinder.
The optical design of rotating mirror-based cameras therefore requires the final image plane to be substantially cylindrical. Historically, in the commercial embodiment of these cameras, the image plane was configured to accommodate 70 mm photographic film, which lends itself to forming a near cylindrical shape very easily and can capture the very high spatial and temporal resolution these cameras are capable of producing. During the time that photographic film was the standard for image capture and widely available, this embodiment satisfied the needs of the application of this technology.
Presently, electronic imaging technology has supplanted photographic film. The imaging performance of the highest-end electronic sensors has matched, and in some cases surpassed, that of even 70 mm film in terms of resolution and dynamic range. Commercially available devices based on complementary metal-oxide-semiconductor (CMOS) and charge-coupled devices (CCD) are a viable replacement for photographic film in most imaging applications.
Application of CMOS or CCD technology within a rotating mirror streak camera application is not a straightforward replacement. First, the aspect ratio of the final image plane in a rotating mirror streak camera is highly rectangular. Aspect ratios of 10:1 are typical of records previously captured on film. In contrast, commercially available electronic image sensors can have maximum aspect ratios approaching 3:1. The long axis of certain such sensors can posses sufficient resolution (a sufficient number of sufficiently small pixels) to effectively capture streak propagation in a streak-length direction. However, commercially available sensors simply do not possess sufficient spatial resolution in the short axis to be a viable direct replacement for film. That is, the short axis does not possess sufficient resolution and length to record a continuous streak image for a sufficient length of time.
Second, the image capturing surface of a commercially available electronic image sensor is planar due to conventional and cost-effective fabrication methods. Since the image plane of a rotating mirror streak camera is approximately cylindrical, a simple replacement of the film capturing medium with a single (and currently hypothetical) planar electronic imaging device that does provide the desired aspect ratio in a single sensor would not provide sufficient agreement between the image plane defined by the optics of the camera and the image capturing surface of the hypothetical electronic sensor. Consequently, the image captured by such an image sensor would not be in focus during the entire desired time period.
In theory, a plurality of high resolution commercially available sensors could be placed side-by-side to dispose their short axes in a segmented, generally arcuate path that approximates the image plane of the camera, such that a desired spatial (time) resolution requirement can be met in series. However the image plane is also necessarily continuous. Commercially available electronic image sensors are fabricated with required supporting edge structures that prevent the image-capturing surfaces of two adjacent image sensors to be positioned in perfect abutment. The resulting unavoidable spacing between adjacent image-capturing surfaces makes it impossible to simply place sensors side-by-side in the image plane and thereby capture an image without discontinuities and lost image information.
Furthermore, alternatives to such sensors would be extremely expensive to manufacture in the required arcuate configuration and extended length and width to directly replace the length of film used in a conventional streak camera.
It would be an advance in the field of image capturing to provide an apparatus that can produce a one-dimensional, time-based image as a digital image, with the advantages of immediate availability, easy duplication, modification and analysis, and convenience endemic to digital imaging in general. It would be a further advance to provide an apparatus that incorporates a plurality of commercially available, reasonably priced, image-capturing sensors in an arrangement effective to capture a virtually uninterrupted image from the entirety of an image plane that is at least approximately equivalent in spatial and temporal resolution, and in dynamic range, to a known streak camera adapted to use photographic film.